1. Field of the Invention
This invention relates to a packaging structure in which electronic components are mounted on a substrate. With respect to such a packaging structure, for example, thermal mass flow meters, thermal current meters, Pirani gauges, light-emitting element packaging structures and packaging structures of an electronic component such as a large-capacity capacitor are listed.
2. Description of the Related Art
Upon packaging an electronic component such as a sensor on a substrate such as a printed-circuit board, normally, the electronic component is mounted on the surface of the substrate. For this reason, the substrate of the packaging structure comes to have a surface with irregularities.
For example, as a thermal flow sensor used for measuring a flow speed or a flow rate of a fluid, such a flow sensor has been known in which a heat-generating resistor forming a flow-speed detecting resistance and a temperature measuring resistor forming a temperature compensating resistance are formed on a surface of an insulating layer integrally formed on the surface of a silicon substrate. In the thermal flow sensor of this kind, the silicon substrate under the heat-generating resistor is removed by a means, such as anisotropic etching, so that a space portion is formed therein; thus, the rear face of the insulating layer is exposed so that the heat-generating resistor and the silicon substrate are thermally insulated from each other (see Japanese Unexamined Patent Publication No. 9-133563).
In the thermal flow sensor, although the space portion is formed on the rear face side of the substrate, both the heat-generating resistor and the temperature-measuring resistor are formed on the surface of the substrate, and are not formed in the space portion.
Here, since electronic components such as sensors mounted on the substrate surface can be easily touched, those components that are vulnerable to damage when touched tend to have a higher possibility of malfunction.
The substrate surface is not flat when electronic components have been mounted thereon; therefore, in the case when a component that is adversely affected in its performances upon receipt of a stress is mounted, it is highly possible that the performances of the resulting packaging structure become unstable.
Moreover, upon packaging a substrate on which electronic components have been mounted on a pipe-shaped member having a limitation in its space, those components having a large volume are not adopted in some cases. As a result, instead of using an inexpensive component having a large volume, a component that has a smaller volume, but is expensive, needs to be mounted to sometimes cause high costs.